BETting on Th17 cells

Researchers at Constellation Pharmaceuticals Inc. have established a connection between BET bromodomain proteins
and pathogenic T helper type 17 cells.1 The findings hint at therapeutic
opportunities for this epigenetic target class in autoimmunity and inflammatory
disease, but further details about tolerability and specificity of current BET
inhibitors are needed.

BET
bromodomains are protein modules that bind to acetylated lysine residues of
histones, the principal components of chromatin. Small molecule BET inhibitors
mimic acetylated lysine and prevent BET proteins from recognizing chromatin,
leading to transcriptional changes that arrest cell proliferation and
differentiation.

Most
efforts to target BET proteins have focused on cancer. BET inhibitors from
Constellation, GlaxoSmithKline plc and Oncoethix S.A. are in Phase I
testing for a range of tumor types.

In
2010, an international team that included The Rockefeller University and GSK reported that GSK525762, a broad-spectrum BET inhibitor
in Phase I testing for epithelial cancer, also had potent anti-inflammatory
effects in macrophages.2 Rab Prinjha, VP and head of the EpiNova
epigenetics discovery performance unit at GSK, said that results of the trial
are expected in 2014.

Now,
a Constellation team has shown that BET proteins influence the development and activation of T helper type 17 (Th17) cells,
which are dysregulated in autoimmune diseases including multiple sclerosis (MS)
and rheumatoid arthritis (RA).

"There
was already a good notion that this family of proteins controls inflammation,"
said Jose Lora, team leader and executive director of preclinical sciences at
Constellation. "The novelty of our recent paper is that these BET proteins
control an important differentiation pathway in T cells."

Lora
previously was director and head of immuno-epigenetics biology at GSK.

He
said that the findings suggest BET proteins regulate inflammation by
controlling expression of Th17 cell-specific proinflammatory cytokines.

"We
are showing that this pathway is a fundamental mediator of transcription of
these cytokines and show in animal models that this is a reasonable entry point
for therapeutics," he added.

Lora's
team treated undifferentiated human T cells with JQ1, a broad-spectrum, small molecule
BET inhibitor identified by researchers at Harvard Medical School,3 and
then subjected the cells to conditions that promoted differentiation of various
helper cell subtypes.

JQ1
prevented differentiation of Th17 cells but not of Th1, Th2 or Treg
cells, whereas vehicle had no effect.

The
team obtained a similar result in actual Th17 cells. That finding suggests that
BET blockade not only could block the initiation of the Th17 cell response but
also potentially could shut down active Th17 cells.

In
mouse models of MS and RA, JQ1 decreased Il-17 production and disease severity
compared with vehicle.

Results
were reported in The Journal of Experimental Medicine.

Helping the
helpers

Although the findings provide proof of
concept for therapeutic intervention with BET inhibitors in Th17 cell-driven
autoimmune diseases, the precise role of BET proteins in autoimmunity is still
up for debate.

For
example, the new findings contrast with last year's report in the Proceedings
of the National Academy of Science from a team led by Anjana Rao that found
that GSK525762 prevented the initiation of the Th1 cell response but could not
halt ongoing autoimmune activity.4 Rao is a professor of signaling
and gene expression research at the La Jolla Institute for Allergy & Immunology.

In
that study, GSK525762 blocked "the initial phases of T cell
differentiation with specific and selective effects driven in a
context-specific manner by signature cytokines more in Th1 than Th17 cells,"
said Prinjha, who was a coauthor of Rao's study.

Prinjha
thinks that methodological differences between the two experiments could
account for the divergent conclusions. He said that Rao's team used adoptive T
cell transfer for its in vivo work, whereas Lora's team did much of its
work in vitro in human T cells and in mouse disease models with
relatively unmanipulated immune systems.

Alexander
Tarakhovsky, a professor in the laboratory of immune cell epigenetics and
signaling at Rockefeller University, said that the new findings underscore the
limited ability of animal models to capture the full complexity of autoimmune
disease.

"Current
models of autoimmunity typically focus on a particular type of immune response,
but such studies are not sufficiently inclusive of other aspects of immunity,"
he said.

Thus,
said Tarakhovsky, the two teams' conclusions "are not mutually exclusive
and could result from different experimental setups."

Tarakhovsky
led the team that first reported the anti-inflammatory effects of BET
inhibitors.2 He is collaborating with GSK to characterize the
immunological effects of the company's BET compounds.

Lora
concurred that "the difference in models probably explains the differences"
between the two studies.

Nevertheless,
it is not certain that blocking Th17 cell activity is the main effect of BET
inhibitors on the immune system.

"It
is still unclear what is the most important site of action of the BET
inhibitors," said Prinjha. "Our adoptive transfer experiments in the
Rao paper went some way to showing the importance of Th1 cells, but more work
is needed to characterize other immune cell types."

Hozefa
Bandukwala, first author on last year's PNAS paper and now a principal
scientist at Pfizer Inc., said that BET inhibitors likely affect a
broad range of immune cells besides T helper cells.

Red light

Tarakhovsky said that uncertainty about what
exactly BET inhibitors do to immune function should serve as a warning against
leaping into the clinic.

He
said that BET proteins appear to affect the development and activity of
multiple immune cells, and thus he thinks that current BET inhibitors are
likely to have broad immunosuppressive effects.

Current
BET inhibitors are relatively unselective and are likely to hit multiple
members of the BET protein family.

"In
fact, we know very little about the true spectrum of the effects of these
compounds," he said.

Improving
the selectivity of BET inhibitors is a logical way to help position the
molecules in autoimmune diseases.

Humans have at least nine BET proteins, the best
characterized of which include bromodomain
containing 2 (BRD2)
and BRD4.
Many other proteins feature bromodomains that are potentially affected by BET
inhibitors.

"One potential area for improving the safety profile
of BET inhibitors is to narrow the specificity to BRD2 and BRD4, but because
all bromodomains are very similar, it will be challenging to achieve
selectivity for one over the other," said Lora.

Lora added that Constellation has a discovery-stage
program to identify selective BET inhibitors for autoimmune and inflammatory
disease. The company's lead product in the space, CPI-0610,
is in Phase I testing for lymphoma. Constellation did not disclose details
about that compound's selectivity.

Meanwhile,
genetic methods could help determine which BET proteins are working in which
immune cells to drive autoimmunity.

"To
get an answer about which specific proteins and cells are involved in which
aspects of autoimmune disease, we would need conditional genetic knockouts,"
said Bandukwala.

Lora
and Prinjha said that ongoing Phase I trials of pan-BET inhibitors in oncology
should lead to insights about the immunological effects of blocking this
protein family.

"The
ongoing trials in oncology are going to be very informative. This is the first
time that we'll see the safety profile of these compounds and how well they're
tolerated," said Lora.

"I
suspect the final choices of immuno-inflammatory disease indication will be
driven by data that emerge from our ongoing clinical studies rather than more
preclinical work," said Prinjha.

Constellation
did not disclose whether it has filed patents in connection with the JEM study.

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